774 ELECTRICAL METHODS [Chap. 10 



Electromagnetic-inductive methods may be divided into horizontal- and 

 vertical-loop methods. The choice between them should theoretically be 

 controlled by the closeness of coupling (loop and geologic body in parallel 

 planes). For instance, in exploration of stratified ground, the loops should 

 be horizontal ; in prospecting for vertical or steeply dipping ore bodies, the 

 loops should be vertical. However, a limitation is placed on this pro- 

 cedure, since large vertical loops are diflficult to handle and since, with 

 practical sizes, their range is comparatively small. Hence, horizontal loops 

 are used more extensively. The following frequency ranges are applied in 

 inductive methods: (1) low frequencies (30 to 100 cycles), (2) audio fre- 

 quencies (250 to 1000 cycles), and (3) high frequencies (several tens of 

 kilocycles). 



1. Horizontal-loop methods, (a) Power supply. A large variety of 

 transmission and generating equipment is available for use with insulated 

 loops. A simple procedure is to feed the loop from the industrial power 

 network through a suitable rheostat and transformer. This method has 

 been applied where ore veins, fault conditions, and the like, were investi- 

 gated m or near electrically operated mines. As a rule it is preferable to 

 employ frequencies that are removed from the commercial frequencies to 

 avoid interference. Gasoline-engine driven generators provide ample 

 power (j to ^ kw^.) for this purpose. It is desirable to provide them with 

 a frequency meter or frequency bridge.*^ For absolute intensity measure- 

 ments with a vacuum tube voltmeter, great constancy of output and 

 frequency are required. 



Although of low power (15 to 20 watts), storage battery operated buzzers 

 are satisfactory for moderate depths. Two types are illustrated in Fig. 

 10-97. Motor generators have been applied with storage battery driven 

 (6 or 12 volt) D.C. motors and A.C. generators of low or audio frequency. 

 Vacuum tube oscillators are used where both constancy of a given fre- 

 quency and adjustability of frequency are desirable. Their output can be 

 increased to about 80 watts when they are used with a power amplifier. 

 The oscillator shown in Fig. 10-98 may be adjusted in frequency between 

 5 and several hundred cycles and has an output of several watts. In the 

 power amplifier shown in the same figure, the plate supply is furnished 

 from an A.C. generator, rectified, filtered, and connected to the center tap 

 of the output transformer and the cathodes of six screen-grid tubes. 



(b) Transmission units. Transmission imits in inductive prospecting 

 have the form of extended lines, rectangles, or circles. Single cables and 

 rectangles are arranged parallel to the strike and profiles are surveyed at 

 right angles to the strike. 



" Canad. Geol. Survey Mem., 165, 150. 

 «8 Muller, Gerl. Beitr., 21(2/3), 241 (1929). 



